While this isn't really a technical question related to this form, I think it is a really good question. How I would start is to use the power of social media. Begin by making a Google Page and Google Group. If you have people/members in your area that you would like to be apart of it, send them join requests. Now you will have some formality, and a means to communicate.
One thing you will want to do is identify the mission and purpose of the group. Are you looking to share ideas, go after work, get grants, save the world? I would suggest setting up an organizational structure to your group. If you are planning on being formal in any manor, you will want to have semi regular meetings, with an agenda. I would structure some roles within your group, such as president, VP, treasurer (if you plan to deal with money), and an individual to take meeting notes.
After this, there are lots of things you can do, but it will depend on the goal of the group. You may want to make ties/connections with local organizations, such as schools, college, university, government, business, environmental agency, etc. But again, it depends on the goals of the group.